This invention relates to a method for polishing diamond. More specifically, it relates to the use of plasma-enhanced chemical etching techniques for polishing synthetic diamond to an optical quality surface.
Extreme hardness, high thermal and chemical stability, and optical transparency are properties that render diamond desirable in numerous optical, electrical, and military applications. In order to overcome the rarity and cost of natural diamond, synthetic methods of diamond preparation have been developed. Synthetic diamond is efficiently and cost-effectively fabricated in the form of coatings using plasma-assisted (or plasma-enhanced) chemical vapor deposition (PACVD or PECVD) processes. As deposited, the diamond films are polycrystalline, typically possessing a roughness on the order of 10 to 20 micrometers. The rough surface negates the utility of synthetic diamond in many applications, particularly optics. When the as-prepared synthetic diamonds are used as lens coatings, for example, their rough surfaces produce excessive scatter and thus, provide low transmittance. Costly, labor-intensive polishing must be performed in order to achieve the required finish for this type of application. Mechanical polishing techniques utilizing diamond paste are typically performed, but because the synthetic diamond has the same hardness as the diamond in the paste, polishing must be performed repetitively, and for an extended period. As a result, the cost of polishing the diamond to optical quality exceeds the cost of depositing the diamond layer.
To reduce the polishing time and cost, a repetitive ion-implantation mechanical polishing technique was designed and is disclosed in U.S. Pat. No. 5,154,023. When utilizing this technique, the rough diamond surface is first xe2x80x9csoftenedxe2x80x9d by the formation of an ion-implanted layer. This softened layer is subsequently subjected to mechanical polishing. The softening and polishing steps are repeated until a desired surface smoothness has been achieved. Each cycle of softening and polishing in this technique affects only a shallow surface layer (on the order of 0.1 microns), so dozens of cycles are necessary to process typical synthetic diamond to optical quality. The repetitive ion implantation polishing technique requires high ion energies (on the order of 100 keV) in order to achieve ion implantation in the diamond surface; this requirement contributes both to overall cost of the method and also raises potential safety issues. Because the synthetic diamond surface has various grain orientations, line-of-sight effects from the high-energy implantation can result in directional sputtering on the surface, thus hampering the production of a smooth surface. In addition, the ion implantation apparatus typically has a small beam spot and therefore, repeated scanning of the beam over the sample is necessary to achieve uniform ion-implantation across the surface of a large synthetic diamond.
Synthetic diamond films and wafers have been used in various microelectronic applications, such as heat sinks or substrates for semiconductor devices. In these applications, it is often desirable to impart a specific architecture on the surface of the diamond. Oxygen plasma, coupled with pattern masking, has been utilized to selectively etch synthetic diamond wafers. Typically, the masked wafer is etched in a low-pressure oxygen gas reactor, using electromagnetic radiation to generate an oxygen plasma. Under these conditions, etching of the diamond wafer is rapid compared to conventional mechanical polishing techniques. However, under these conditions, the etching of the wafer is anisotropic, most likely due to physical bombardment by high-energy molecular oxygen ions, so polishing to optical quality smoothness is not feasible.
One object of the present invention is to overcome the disadvantages of the methods described above by providing a method for rapid, uniform, and cost-effective synthetic diamond polishing. More specifically, one embodiment of the present invention provides a method of effectively polishing synthetically produced diamond by plasma-enhanced chemical etching using an atomic oxygen polishing plasma source, said source generating high concentrations of low energy atomic oxygen ions over a large surface area. The present invention takes advantage of the ability of low energy atomic oxygen ions to chemically etch a diamond surface at moderate temperatures. Because the atomic oxygen ions have low energy and high density, they conform to the surface of the synthetic diamond sample, and thus polish the sample with increased uniformity versus known oxygen etching techniques. The rate of polishing is proportional to the density of atomic oxygen, and, in the present invention, this density can be easily controlled by adjusting parameters such as gas pressure, discharge voltage, and plasma ion source power to minimize the processing time.
Because the present invention utilizes a chemical effect to polish the diamond surface, the atomic oxygen ions generated are of much lower energy than ions generated for ion implantation techniques or similar high energy beam approaches. Accordingly, the atomic oxygen polishing plasma source disclosed herein can operate at lower voltages than the apparatus for ion-implantation, thereby reducing both capital investment and safety concerns. In addition, because the atomic oxygen polishing plasma source generates a large plume of plasma, large diamond samples can be polished in their entirety without beam scanning, and multiple samples can be polished simultaneously.